History of linacs - page 1

      The history of particle accelerators for ion beams is often described in association with the development of cyclotrons, primarily due to their wide-spread use in the medical field. However, what is often not acknowledged is that ion linear accelerators ("linacs") were developed in parallel with the cyclotron and other circular accelerators. While Lawrence and Livingston designed the first small cyclotron in 1930, R. Wideröe had already published a paper in 1928 on his results from an rf powered linear accelerator for ions. This device, shown schematically in figure 1, followed a proposal in 1925 by G. Ising. It consisted of a series of cylindrical tubes, placed along the longitudinal axis of an evacuated glass cylinder. Alternate tubes were connected to opposite terminals of an rf generator. By selecting the frequency and applied rf voltage, a variety of heavy ions could be accelerated across the gaps and bunched simultaneously. The ion velocity fits the criterion that the distance between the midpoint of each gap is given by ½ ν /c · c/f ≡ ½ β λ . The Wideröe design is a standing wave cavity in which the accelerating electric field maxima and nodes remain fixed in space. At a moment when the rf voltages are maximum on each electrode, acceleration takes place in every other gap. After crossing these gaps, the particles (ions) enter the metallic cylindrical electrodes which act as a shield to the electric fields. These particles then exit the electrode after the field in the next gap has changed polarity. The length of these cylindrical electrodes must become longer as the particles are accelerated to make the beam arrive at the next gap when the rf voltages are appropriate for accelerating. Sloan and Lawrence used such an array of 30 electrodes excited with a voltage of 42 kV at an rf frequency of 10 MHz in 1931 to accelerate Hg⁺ ions to 1.26 MeV.